Analyzing an inorganic sample via analytical techniques such as x-ray fluorescence (XRF), inductively coupled plasma (ICP), atomic absorption (AA) requires that the sample be specially prepared before analysis. The sample must often be in the form of a homogeneous, solid, smooth-surface shape, such as that of a disk or bead. In this form, the sample does not exhibit mineralogical, grain-size, or orientation effects that might otherwise skew the analytical results.
A process known as “fusion” can be used to prepare samples for XRF, ICP, and AA. During the fusion process, a powdered sample is dissolved in a solvent, typically a lithium borate flux. The flux is solid at room temperature and must therefore be liquefied, which typically occurs at high temperature (c.a. 900 to 1000° C.). After complete dissolution of the sample, the molten solution is poured into a plate-shaped platinum mold. Cooling results in a small, homogeneous glass-like disk or bead of sample, now suitable for analysis.
As a consequence of the high temperatures required, the fusion process is performed in a heater. Energy for the process is supplied either by gas (i.e., a gas heater) or electricity (i.e., an electric furnace). Electrically powered furnaces can be inductive or resistive. Resistive furnaces offer the best temperature stability and accuracy.
FIG. 1 depicts the salient elements of prior-art resistive fusion furnace 100. Furnace 100 comprises inner walls 102, door 104, heating elements 110 (only one is visible in the view shown), and power control system 112. Inner walls 102 define furnace cavity 106. Power control system 112 includes temperature sensor 114, controller 116, and switching device 118.
A “platinumware” assembly (not depicted) is used in conjunction with furnace 100. The platinumware assembly includes a crucible holder, which supports a plurality of platinum crucibles, and a mold rack, which supports a like number of platinum molds. The assembly is arranged to slide in and out of furnace cavity 106. Once the flux and sample are deposited into the crucibles, the assembly is moved into cavity 106 and door 104 closes to begin the fusion process. See, e.g., http://www.katanax.com/cgi/show.cgi?products/K2prime/K2primevideo.l=en.html.
Furnace 100 includes a plurality of heating elements 110. In the embodiment depicted in FIG. 1, heating elements 110 are arranged vertically along the back wall of inner cavity 106. The heating elements are typically arranged in arrays (e.g., 3×1, 5×1, etc.). Although arranged vertically in FIG. 1, in some other embodiments, the heating elements are arrayed transversely. Also, in some other embodiments, rather than being arrayed against the back wall of the inner cavity, the heating elements can be disposed along the upper and/or lower wall of furnace 100, in either a front-to-back or transverse orientation. Electrical leads of each heating element 110 electrically couple it to a source of electrical energy.
The temperature of each heating element rises as electrical energy is delivered thereto. The heating elements comprise an electrically resistive filament capable of tolerating high (typically up to at least 1200° C.) temperatures. The resistive material can be, for example, and without limitation, tungsten, molybdenum, tantalum, niobium, rhenium, osmium, carbon, or any combination thereof; it can also be a compound such as silicon carbide (SiC), silicon nitride (Si3N4), molybdenum disilicide (MoSi2), or other alloys, such as iron-chromium-aluminum (FeCrAl).
Any gas that is produced during the fusion process is exhausted through vent 108. Furnace 100 often includes cooling capability, such as a fan (not depicted), to prevent the connection point of heating elements 110 (located outside of furnace inner cavity 106) from overheating.
Closed-loop feedback, as implemented by power control system 112, controls the temperature within furnace cavity 106. Temperature control by conventional power control system 112 is discussed in more detail in conjunction with FIG. 2.
FIG. 2 depicts further detail of conventional resistive fusion furnace 100; in particular, details of the power-control system 112 are shown. For pedagogical purposes, furnace 100 is depicted as having five heating elements 210A, 210B, 210C, 210D, and 210E (collectively, heating elements 210), although furnaces with fewer or greater numbers of heating elements are commonly commercially available.
In the typical implementation of power control system 112 depicted in FIG. 2, temperature sensor 114 is embodied as thermocouple 214 and switching device 118 is implemented as switch 218. Electrical energy from an energy source (not depicted) is delivered through switch 218 to heating elements 210. Controller 116 controls the electrical energy flowing to heating elements 210 by opening and closing switch 218. Typically, controller 116 causes switch 218 to close when the temperature measured by thermocouple 214 falls below a desired operating temperature (i.e., set-point temperature), and to open when the temperature measured by thermocouple 214 rises above the set-point temperature. Other methods for controller 116 to control the open-or-closed state of switch 218 are well known in the art.
Typically, furnace 100 is designed such that, when switch 218 is closed, heating elements 210 receive sufficient electrical energy to ensure that the temperature in furnace cavity 106 will reach any desired set-point temperature. To maintain the temperature in the cavity near the set-point temperature, controller 116 implements a feedback control loop by appropriately cycling switch 218 “on” and “off” as described in the previous paragraph. This basic form of temperature control is essentially the same as employed in a simple home thermostat.
There are inherent manufacturing variations in heating elements, such as heating elements 210A, 210B, 210C, 210D, and 210E. For example, depending on the material and manufacturing process, a given heating element will have an actual electrical resistance that can vary significantly (up to +/−20%) from the nominal value. Thus, even if heating elements 210 are all specified to have a nominal power rating of 800 watts at a specified nominal voltage and temperature, some of them might receive an amount of power as low as about 640 watts while others receive an amount of power as high as about 960 watts, when connected to the nominal voltage at the nominal temperature. This means that some heating elements will heat to a significantly higher temperature than other heating elements. The disparity between heating elements might even be worse at voltages and temperatures other than the nominal.
As a consequence, even if controller 116 is able to maintain an average cavity temperature, as measured by thermocouple 214, close to the set-point, the temperature profile across furnace cavity 106 is likely to vary significantly.
In addition to the aforementioned manufacturing variations, heat loss through the furnace walls 102 affects the temperature profile in furnace cavity 106. In particular, even with transversely (i.e., left-to-right) oriented heating elements in a closed furnace, the left- and right-most positions within furnace cavity 106 are likely to be somewhat cooler than the center positions. And the center position is likely to be hotter than all other positions. This results in a temperature profile that peaks toward the center of the furnace cavity.
This position-based temperature profile is likely to exacerbate temperature variations that result from the aforementioned manufacturing variations in heating elements 210. The temperatures across furnace cavity 106 are therefore likely to vary significantly because of these issues, which can ultimately bias the final analytical result due to inconsistent reaction or evaporation of the prepared sample.